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Full text of "Handbook Of Chemical Engineering - I"

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PYROMETRY
429
set at 14 the pointer would have read 0.42, and the final reading would be 14.00 — 0.42 = 13.58 as before.    To avoid the necessity of thus subtracting a number from the dial setting, the galvanometer scale may be graduated from left to right as follows: center
0,     10,------80,    90,     0,     10,------100.    When the pointer swings to the left the next
lower dial reading is used. Hence in the above example with the dial set at 14, the galvanometer would read 58 on the left part of the scale. Thus the correct value is 13.58 as before. In many industrial processes the temperature of the couple will vary only slightly during several hours, so that a new dial setting is infrequently required. Thus the method for obtaining the electromotive force of the couple is nearly as simple as when an ordinary galvanometric indicator is used.
The theory of the deflection potentiometer has been developed in detail by Brooks,1 and instruments of this type have been designed by Brooks and by White. A compensating resistance is mounted as an integral part of the dial so that turning the dial
FIG. 8.—Deflection potentiometer.
changes the electromotive force setting and at the same time adjusts the compensating resistance in the galvanometer circuit to its proper value.
Figure 8 illustrates a deflection potentiometer for thermocouples, made by the Taylor Instrument Cos. This instrument is known as the "range control board." The galvanometer G is provided with two scales, in the ranges zero to 500°C. and 450 to 950°C. respectively. The galvanometer circuit is connected at fixed points a and b within the potentiometer, and when the instrument is to operate in the lower range the battery circuit is opened. Thus the potentiometer setting ef is made zero without changing the value of r\. In this range the instrument operates as an ordinary galvanometric indicator. If the temperature of the thermocouple is about 450 or 500°C., a current of such magnitude is made to flow through n that the potential drop <?' across n balances the electromotive force developed by the couple when at 450°C. The temperature will then be indicated on the high range scale.
The total resistance of the galvanometer circuit is almost exactly equal when operating in either range, since when operating in the upper range the shunting effect of rz -f r3 on TI is negligible. Since a separate galvanometer scale is provided for each setting of the potentiometer (o and e'} it is not really necessary, however, that the sensitivity be equal in the two cases.
For the high range, the current from the battery is adjusted by connecting switch S as shown by the dotted line and setting r3 so that the galvanometer deflects to a
i Bureau of Standards Sci. Papers Nos. 33, 79, 172, 173